The effect of molecular architecture on the deformation behaviour of drawn bimodal polyethylene

The most common failure mode for polyolefin pipes is slow crack growth. A crack is preceded by a craze, a voided wedge of material bridged by highly deformed fibrils. Upon failure of the fibrils, the crack propagates. Both the tendency of the material to form voids and the strength of the fibril at the craze - crack interface are governed by the effective entanglement network. The effective entanglement network comprises all the intermolecular junctions in the material that can effectively transfer load at the time scale of the experiment. In this work, the effective entanglement network of bimodal polyethylene is probed through tensile and creep measurements. Bimodal polyethylene is the industrial standard material for polyethylene pressure pipes, and consists of a high molecular mass, branched fraction and a low molecular mass, linear fraction. The former is responsible for the resistance to slow crack growth, the latter for enabling processing. In the first part of the work, the influence of molecular mass and branch content of the high molecular mass fraction on the effective molecular network is studied. It is found that only a combination of high molecular mass and high branch content increases the resistance of the network. In the second part of the work, the high molecular mass fraction of the bimodal polyethylenes is isolated. Again, a combination of high molecular mass with high branch content results in a higher effective entanglement network, and overall the resistance to deformation is higher in these materials than in the bimodal materials. It is concluded that the resilience of the network depends on the available network density and the friction caused by side chain branches. Independently of the morphological origin of this friction in the solid material, it can be expected to vary with the monomeric friction in the melt.